Amifostine is a phospho ester compound that is used pharmacologically for the amelioration of side effects of radiation and chemotherapy. The compound is currently marketed as a lyophilizate of a crystalline trihydrate of amifostine, typically in vials of 500 mg, which is reconstituted to a final administration concentration of 5 to 40 mg/ml for IV infusion. Reconstitution procedures leave room for the possibility of not really dissolving all of the lyophilizate and therefore potentially underdosing the patient in question, a result which does not frequently occur, but is unacceptable when it does. Amifostine is known to undergo hydrolysis in aqueous solution at acidic, neutral, and mild basic conditions. Thus, it was believed that stable aqueous formulations that would not hydrolyze to any appreciable extent over typical storage time periods were not likely to be found. Furthermore, the aqueous solubility of amifostine trihydrate in water is about 200 mg/ml at ambient conditions, and further, when such a solution is stored at refrigerated conditions to thermodynamically limit the hydrolysis, substantial crystallization results. In addition, because of limited refrigeration space, kinetic and neutralization/dilution considerations, substantially less concentrated concentrates are not suitable either since they take up additional space and the lesser the concentration, the less stable the concentrate. Thus, aqueous liquid concentrates of amifostine were simply thought not possible. Because such an aqueous liquid amifostine concentrate would eliminate a potential source of dosing errors in clinical practice, there is clearly a need for the same.
There are four pKas for amifostine. The Merck Index lists them to be <2.0, 4.2, 9.0 and 11.7. The amino groups (2) and the phosphate group (2 pKa per group) can all be completely protonated. [Ref: Structure of Radio protective agent S-2(3-Aminoprpropylamino)ethylphosphorthioic acid (WR 2721) Jean Karle, Acta Cryst (1988) C44 135-138]. The usual form crystallized from solution is a double zwitterion with the phosphate completely dissociated (−2) and the amino groups fully protonated (+2). It is usually drawn with the amines in their free base form and the phosphate completely protonated but that is not how it exists in solution. So, the two pKa values from the double zwitterion are for the two amino functions ionizing (i.e., pKas 3 & 4). The first two pKas are for the phosphate which are perturbed by the two positive charges on the amino groups which pushes them lower than pKa1 and pKa2 for phosphoric acid. The following table describes the ionic nature of the compound as function of pH according to the pKa values reported in the Merck Index. The table also summarizes the approximate concentration of ionic species as function of pH:
pKapKa 9.0pKa 9.0pKa 11.7pKa 11.7Prim. aminePrim. amineSec. amineSec. aminepHpKa <2.0pKa 4.2ProtonatedUnprotonatedProtonatedUnprotonated7PO3 (−)PO3 (−)~100%(+) ~0%~100%(+)~0%9PO3 (−)PO3 (−)~50%(+)~50%~100%(+)~0%10PO3 (−)PO3 (−)~10%(+)~90%~100%(+)~0%11PO3 (−)PO3 (−)~1%(+)~99%~83%(+)~17% 12PO3 (−)PO3 (−)~0.1%(+)>99.9%  ~33%(+)~67% 13PO3 (−)PO3 (−)~0.01%(+)99.99%   <5%(+)>95% With increasing pH, the concentration of the amine free base increases for both amine functions. For example, at pH 11 the primary amine with pKa 9 is predominantly present as the free base form. At pH 13, both primary and secondary amines are predominantly present as their free base forms.